Method for the anodization and resin-coating of aluminous articles

ABSTRACT

A method for treating the surface of an aluminous article is provided which comprises conducting the electrolysis with the aluminous article as the anode and in a single electrolytic bath containing 1. AT LEAST ONE STRONG ACID KNOWN PER SE FOR THE ANODIZATION OF AN ALUMINOUS MATERIAL; 2. AT LEAST ONE POLYBASIC ORGANIC ACID; AND 3. AT LEAST ONE CATIONIC TYPE RESIN PREPOLYMER OR PRECONDENSATE IN AN AQUEOUS SYSTEM WHEREIN SAID CATIONIC TYPE RESIN PREPOLYMER OR PRECONDENSATE IS ANIONIZED AT LEAST PARTLY, SO THAT THE ANODIZATION AND ELECTRODEPOSITION OF THE RESIN PREPOLYMER OR PRECONDENSATE ARE EFFECTED ON THE SURFACE OF THE ALUMINOUS ARTICLE IN SAID SINGLE BATH.

United States Patent; 1191 Suematsu METHOD FOR THE ANODIZATION ANDRESIN-COATING OF ALUMINOUS ARTICLES [75] Inventor: Yasuo Sueniatsu, Amagasaki, Japan [73] Assignee: Shinto Paint Co., Ltd., Amagasaki,

Japan [22] Filed: Mar. 22, 1972 [21] AppL No; 236,988

[30] Foreign Application Priority I Data 3,223,607 12/1965 Millner.etal. 204/181 Aug. 28, 1973 2/-1970 Wiegel 204/l8l 6/1972 Terai et a1.204/l8l 571 ABSTRACT A method for treating the surface of an aiuminousarticle is provided which comprises conducting the electrolysis with theaiuminous article as the anode and in a single electrolytic bathcontaining 1. atleast one strong acid known per se for the anodizationof an aiuminous material; 2. at least one polybasic organic acid; and 3.at least one cationic type resin prepolymer or precondensate in anaqueous system wherein said cationic type resin prepolymer orprecondensate'is anionized at least partly, sothat the anodization andelectrodeposition of the resin prepolymer or precondensate are effectedon the surface of the aiuminous article in said single bath.

I 11 Claims, No Drawings larly aluminum base alloy which is capable ofbeing anodized. It is known to produce a porous film or layer on thesurface of analuminous article by conductingthe electrolyticanodizationwith the use of an electrolyte containing sulfuric acid,chromic acid, oxalic acid or the like. i

It is also known to apply a resin coating on the surface of an aluminousarticle by the so-called electrodeposition. In such electrodeposition ithas been conventional to employ an anionic type resin which showsbehavior as anions in an aqueous bath of paint and is deposited on theanode by passing a current through .the' bath, or a cationic type resinwhich showsbehavior as cations in an aqueous bath and is deposited onthe cathode upon passage of a current through the bath.

In applying a resin coating by electrodeposition on an anodizedaluminous article it has been considered essential tofirst conduct theanodization in a first bath of an electrolyte, and then wash theanodized article and then conduct theelectrodeposition in a second orseparate bath of paint. Thus there were required two separate baths andtwo separateoperative stages. Further,

between these two separate operative stages it was consideredindispensable to wash thearticle. 1 Therefore it is an object ofthis'invention to effect the anodization and electrodeposition resincoating entire surface of an aluminous article in a single bath and in asingle operative stage.

Other objects of this invention will become apparent from'the followingdescription.

Briefly this invention provides a method for treating the surface of analuminous article which comprises conducting the electrolysis with thealuminous article as the anode and in a single electrolytic bathcontaining (1) at least one strong acid known per se for the anodizationof an aluminousma'terial, (2) at least one polybasic organic acid and(3) at least one cationic type resin prepolymer orprecondensate in anaqueous system wherein-the cationic type resin prepolymer orprecondensate is anionized at least partly, so that the anodization andelectrodeposition of the resin are effected on the surface of thealuminous article'in said single bath. a

This invention is based on our unexpected finding that when a certainacidic electrolyte is selected for the anodization a cationic resinprepolymer or precondensate can be at least partly anionized in suchelectrolyte so that, upon electrolysis with an aluminous article as theanode, bothanodization and electrodeposition of the resin on the surfaceof the aluminous article can be effected in the single electrolytic bathin a single operative stage.

ln carrying out the method of this invention there can be used any kindof cationic type resin prepolymers or precondensates insofar as theseare film-forming. For example, resins having amino-groups (includingprimary amino group, secondary amino group, tertiary amino group andquaternary ammonium group) in the densation of alkanolamines with otherpolyols, polyisocyanates, etc.; amino-terminated polyamide resinsobtained by the condensation'of an excess amount of polyamines withpolybasic acids. These amino groupcontaining resins, when dissolved ordispersed in an acidic aqueous solution, show behavior as cationic ionsand therefore are referred to as cationic type resins. If desired, theamino groups in these resins may be alkylated or quatemized in a monomerwell known in the art.

It is preferable that these resins are used in thermosetting form-Forthis purpose, if necessary, self crosslinkable functional groups may beintroduced into the resin itself or it is possible to mix the resin witha crosslinking agent such as condensatesof formaldehyde with phenol,melamine, urea or the like, epoxy compounds, blocked isocyanates, etc. 1g

. According to this invention these resins are used in the form ofprepolymer or precondensate which is curable .by itself or cross-linkingagent upon the subsequent heat treatment. If desired, a mixture of twoor more of these resin prepolymers or precondensates may be used.

The important feature of this invention is to solubilize or dispersesuch cationic type resin and to anionize the same at least partly in anelectrolytic bath which is useful also for the anodic oxidation(anodization) of an aluminous article. Thus the electrolytic solutionShe used in this invention has two different functions i.e. anionizationof the cationic type resin and anodization of an aluminous article.Forthis purpose an aqueous solution of a mixture. of .two or more acidsis used as the electrolytic solution. r

. More particularly the electrolytic solutionto be used in thisinvention should contain at least one strong acid whichis known per sefor the anodization of analuminous article. The acid is sometimesreferred toas acid (1). Examples of such strong acids are sulfuric acid,sulfaminic acid, sulfophthalic acid, sulfosalicylic acid, etc. Theseacids are useful not only for the anodization (anodic oxidation) butalso for facilitating the deposition over the entire surface of anarticle, i.e. increasing the ability of resin to deposit completely onthe recessed areas'or shielded portions or the inside of the box sectionof the aluminous article as the anode (therein after referred to asthrowing powder). Generally, the concentration of such strong acid inthe electrolytic bath is at least'about 0.05 percent. lt is preferaonlyfor solubilizing and anionizing the cationic resin but also forfacilitating the anodization. Generally, the concentration of suchorganic acid in the electrolytic solution is at least about 0.3 percent.It is preferably however that malonic acid is used in a higherconcentration, i.e. about 5 percent or higher.

The upper limit of the concentration of the acid (1) and acid (2) is notcritical so far as the anodization of an aluminous article is possible.However, since the electrolytic solution contains a resin and thereforemay sometimes become high in the viscosity which is not suitable for theanodization. In such case the acid concentration may be somewhat loweredby diluting the electrolytic bath to such extent under which theanodization can be satisfactorily effected. Thus, the upper limit of thetotal acid concentration may vary depending upon the particularconditions (kinds of acids used,

kind and amount of the resin used, etc.) but it can be easily determinedby those skilled in the art.

The concentration of the resin in the electrolytic bath is generally 140 percent, preferably 3 15 percent by weight.

In carrying out the method of this invention the resin prepolymer orprecondensate may be put into an aqueous electrolytic solutioncontaining both acid (1) and organic acid (2) forionizaion therein.However it is preferable that the resin is first mixed with a part ofthe organic acid (2) to solubilize and ionize the resin and then mixedwith the acid (1) and remaining portion of the acid (2).

When the resin is mixed with the acidic electrolyte it is renderedsoluble or dispersable in water due to the reaction of the resin withthe acid groups. In this case, a small amount of a hydrophilic organicsolvent may also be used in order to facilitate the solubilization ofthe resin. Examples of such solvents are alcohols such as isopropylalcohol, ter-butyl alcohol, glycolethers such as ethyleneglycolmonobutyl ether, diethyleneglycol monobutyl ether, esters such as methylformate, ethyl lactate, and ketones such as methyl ethyl ketone, methyln-propyl ketone.

In the above manner there is obtained a uniform solution or dispersionof the resin in the acidic electrolytic aqueous solution containing boththe strong acid (1) and organic polybasic acid (2). It should be notedthat in such aqueous system the cationic type resin prepolymer orprecondensate is anionized at least partly and shows behavior as anionseven though the resin itself is initially cationic. This invention is toutilize this surprising phenomenon in the simultaneous anodization andresin-deposition.

Thus according to the invention an electrolysis is conducted with analuminous article as the anode in the above prepared resin-containingacidic electrolytic bath. An electric current may be passed through theelectrolytic system in a manner well known in the art of the anodizationof aluminous article. When the electrolysis is conducted the aluminousarticle forming the anode is electrolytically oxidized at the surface toform an oxide film or layer on the surface. This phenomenon is the sameas in the conventional anodization of an aluminous article well known inthe art. The important. feature of this invention is that, at the sametime, the resin prepolymer or precondensate now in the form of anions asexplained above will be deposited on the surface of the anode, i.e.aluminous article to form a resinous film or layer thereon. A furtherimportant feature is that the resinous film or layer once deposited onthe surface of the aluminous article is not substantially dissolved inthe acidic electrolytic solution and is satisfactorily adhered on thealuminous article. Thus, according to this invention both anodizationand electrodeposition of resin can be effected in the same and singlebath in a single operative stage. When a direct current is to be passedany suitable material (e.g. carbon, lead, stainless steel, etc.) may beused as the cathode. It is preferable to conduct the electrolysis whilestirring the electrolytic bath. The anodization occurs first and thenanionized resin is deposited on the anode. After a predetermined periodof time the aluminous article is taken out of the electrolytic bath,washed with water and baked to cure the deposited resinous layer. It issometimes preferable to conduct a certain aftertreatment before baking.Such after-treatment will be explained hereinafter.

Sometimes the cationic type resin is not completely anionized andtherefore shows behavior partly as cations. In such case the cationicresin will be deposited on the surface of the cathode during theelectrolysis so that the efficiency of the cathode will be impared. Inorder to avoid this difficulty, it is preferable to employ asemipermeable membrane. Thus, the electrolytic cell is divided into twoby a semipermeable membrane (e.g. cellophane) through which the acidicelectrolyte can pass but the resin in the form of cations can not pass.In the anode compartment there is placed an acidic electrolytic solutioncontaining the resin while in the cathode compartment there is placed anacidic electrolytic solution not containing the resin. By this measurethe cathode is protected against the deposition of the resin thereon. Inthis case, during the electrolysis, a small amount of the cationic resinwill be deposited on the surface of the membrane but such resin in theform of cations is easily redisolved in the acidic electrolytic solutionand therefore there will cause no trouble.

When an alternating electric current is to be used it is prererable touse the aluminous articles for both cathode and anode. In this case itis strongly recommended to employ an electrolytic solution wherein allthe cationic type .resin is anionized as completely as possible.

The conditions for the electrolysis may be varied depending upon theparticular electrolytic solution. Generally, however, the electrolysisis conducted at a current density of 0.1 A/dm 2 A/dm for 3 20 minutes ata bath temperature of 5 30 C.

After the electrolysis the anodized and resin-coated aluminous articleis taken out of the bath and washed with water and then heated or bakedfor curing the resinous layer. However, as explained hereinbefore, it issometimes preferable to conduct an after'treatment before the heattreatment.

The. after-treatment is conducted by washing the resin-coated articlewith an aqueous solution containing an organic solvent. This treatmentis useful to improve the smoothness and abrasion resistance of the resincoating and also to prevent undesired yellow coloration of the resincoating during the subsequent baking or heat treatment. The organicsolvent to be used here is for example a hydrophilic high boiling pointorganic solvent such as ethylene glycol ether or diethyleneglycol ether.If desired a small amount of a hydrophobic organic solvent may also beadded. Further, a slight amount of a surface active agent such assilicone type surfactant, anionic surfactant, cationic surfactant,nonionic surfactant and amphoteric surfactant may be added. Generally,the concentration of the organic solvent in the aqueous treatingsolution is 1 99 percent, preferably 5 70 percent by weight, while thatof the surfactant is 1 percent by weight or less.

Further when the nitrogen content in the resin is large there is apossibility that the resin coating is colored to yellow during thebaking due to the oxidation of such nitrogen. In order to prevent thiscoloration it is preferable to add a small amount (e.g. 1 percent orless) of an antioxidant (such as phenol derivatives, phosphitecompounds, etc.) to the aqueous solution for the after-treatment. Mostlythe resin prepolymer or precondensate deposited on the aluminous articleis thermo-setting and therefore is cured upon baking or heat-treatment.Such baking may be conducted in a well known manner. Thus, for example,the resinous coating may be baked at a temperature of 120 200 C. for 60minutes.

If it is desired to form a colored resinous coating it is possible toadd a dye or pigment to the electrolytic solution. I

According to the invention the fiber or layer of a resin prepolymer orprecondensate deposited on an aluminous article by the electrodepositionupon electrolysis is not substantially redissolved into the acidicelectrolytic solution. Therefore there is no troublesome problem in thisconnection, and the eletrolytic condition should be taken care tosatisfactorily elTect the anodization. For example, the, electrolyticcondition should be so selected that the anodization occurs first andthen the resin electrodeposition takes place. The proper condition (e.g.kind of resin, composition of the electrolyte, bath temperature, currentdensity, etc.) can be easily determined by a simple preliminary test.

The invention will be further explained by referring to the followingExamples which are given for the purpose of illustration only and notfor limiting the scope of the invention. In these Examples all parts andpercentages are by weight unless otherwise specified.

' EXAMPLE 1 Each of a pure aluminum panel 18 1/2 H and acorrosion-resistant aluminum 'alloy panel 528 was degreased with anorganic solvent and immersed in 10 percent aqueous solution of sodiumhydroxide at 60 C. for 3 minutes, then washed with hot water and thenimmersed in 10 percent aqueous solution of nitric acid for 5 minutes,and then well washed with water.

A commercial Epon resin was reacted with di-npropylamine in 1:1 mixtureof butylcellosolve and isopropyl alcohol to obtain 154 parts of aviscous substance containing 65 percent (as solid) of a resinousprepolymer. To this solution wereadded 200 parts .of 25 percent aqueoussolution of citric acid and 50 parts of butylcellosolve to form auniform solution. To this solution were gradually added 400 parts of 5'percent aqueous solution of oxalic acid and parts of 1.0 percent aqueoussolution of sulfuric acid and then 176 parts of ion-exchanged water wereadded for dilution to prepare an electrolytic solution.

An electrolytic cell made of polyvinyl chloride was divided by acellophane sheet into 9 volume compart-' ment and 1 volume compartment.Into the larger-compartment there was placed the above preparedelectrolytic solution while the smaller compartment was filled' with anaqueous solution containing 5 percent citric acid, 2 percent oxalic acidand 0.2 percent sulfuric acid-The whole solution was stirred andmaintained at a temperature of 20 C.

A cathode made of a stainless steel was put into the smaller compartmentand the aluminum sheet (or aluminum alloy sheet) was put into the'largercompartment as the anode. The surface area ratio between the cathode andanode was approximately 1:1. The distance between the electrodes was 10cm. The constant electrolysis was conducted at a voltage of 10 V. for 10minutes. The current density'varied from 0.5 A/dm. to 0.1 Aldm Duringthis electrolysis there were gradually formed on the surface of theanode an anodized film and resin film. After the electrolysis the anodewas taken out of the cell and baked at 160 C. for 20 minutes to form acontinuous and adherent resin coating (thickness? p. on the aluminum (oraluminum alloy) 4 7 sheet. Before the baking the resin coating waspartly removed and it was observed that the exposed surface layer is ananodized yellowish layer of a thickness of about 0.5 1 p.. I

, EXAMPLE 2 9 8.5 parts of an acrylic copolymer (solid content 65percent) havingdepende nt amino groups introduced by the usualcopolymerization with dimethylaminoethyl acrylate in isopropyl alcoholwere mixed with 24.5 parts of water soluble melamine resin (solidcontent 65 percent) prepared by the methyl-etherization of amelamine-formaldehyde precondensate. To this resinous mixture were added200 parts of 25 percent aqueous solution of citric acid, 400 parts of 10percent In this cell were placed a cathode (lead panel) and an anode(aluminum alloy panel of Example 1) with a distance of 15 cm.therebetween, the surface area ratio being 1:1. Initially the constantvoltage electrolysis was .conducted at 15 V for 5 minutes and then thevoltage was increased-to 30V and the constant voltage electrolysis wasconducted for 5 minutes at this higher voltage. The current densityvaried from 1 A/dm to 0.1 A/dm.

After the electrolysis the aluminum alloy panel (anode) was taken out ofthe cell, washed with water to remove the electrolytic solution and thenimmersed in water containing 15 percent butylcellosolve. Then the sheetwas baked at 180 C. for 30 minutes to obtain a continuous and adherentresin coating (thickness about 10 p.) on the alloy panel. Before thebaking the resin coating was partly removed and it was observed that theexposed surface layer is an anodized light yellowish layer of athickness of about 1 u.

EXAMPLE 3 108 parts of a resinous material solid contentpercent)-prepared by copolymerizingglycidyl methacrylate, butyl acrylateand methyl methacrylate in a conventional manner in butyl cellosolve,followed by the reaction with diethanolamine were mixed with 200 partsof 25 percent aqueous solution of citric acid and 60 parts ofbutylcellosolve to form a uniform solution. To this solution weregradually added 400 parts of percent aqueous solution of oxalic acid and20 parts of percent aqueous solution of sulfuric acid. Then the solutionwas diluted with 212 parts of ion-exchanged water to prepare anelectrolytic solution.

An electrolytic cell of polyvinyl chloride was filled with theelectrolytic solution and the bath was maintained at 25 C. whilestirring. in this cell were placed a cathode (carbon rod) and anode(aluminum panel of Example 1) with a distance of cm. therebetween, thesurface area ratio being 1:1. Initially the constant voltageelectrolysis was conducted at 25 V for 2 minutes, during which thecurrent density varied from 1.2 A/dm to 0.2 A/dm. Then, further constantcurrent electrolysis was conducted at 0.07 A/dm for 6 minutes.

After the electrolysis the anode (aluminum panel) was taken out of thecell and washed with water to remove the electrolytic solution and thenimmersed in an aqueous solution containing 40 percent butyl cellosolve,0.04 percent silicone surfactant, 0.3 percent anti-oxidant (phenolderivative) for 2 minutes. Then the panel was heated at 80 C. for 5minutes and baked for 160 C. for 30 minutes to obtain a smooth,wearresistant resin coating (thickness about 25 u) on the aluminumpanel. When the resin coating was removed it was observed that there hadbeen formed a light yellow-purple colored anodized film of a thicknessof 0.5 1 u. The aluminum panel with this anodized film (the resin layerhaving been removed) was subjected to anticorrosion test (CASS testCopper-Accelerated Acetic Acid Salt Spray Testing ASTM B 368-62T, 3hours) and it was observed that the corrosion is only about 1 mm. indiameter whereas the same aluminum sheet without the anodization of thisinvention shows a corrosion of about 5 mm. diameter.

EXAMPLE 4 123 parts of the resinous material used in Example 1 weredissolved in a mixture of 50 parts of butylce1-' losolve and 400 partsof 5 percent aqueous solution of oxalic acid. To this solution wasfurther added 100 parts of 6 percent aqueous solution of sulfaminic acidand 327 parts of ion-exchanged water to prepare a resin-containingelectrolytic solution.

An electrolytic cell was divided into two in the same manner as inExample 1 and the larger compartment was filled with theresin-containing solution while the smaller compartment was filled withan aqueous solution containing 2 percent oxalic acid and 0.6 percentsulfaminic acid. An aluminum anode same as in Example l and a stainlesssteel cathode were placed in the larger and smaller compartmentsrespectively. The electrolysis was conducted at 25 V for 3 minutesduring which the current density varied from 2 A/dm to 0.1 A/dm. Thenthe constant current electrolysis at 0.1 A/dm was continued for 10minutes. The anode was taken out of the cell, washed with water andbaked at 160 C. for minutes to obtain a continuous, tough and adherentresin coating (thickness 10 pt) on the surface of the aluminum panel.The anodized film beneath the resin coating had a thickness of about 11,.

EXAMPLE 5 154 parts of a resinous material (solid content 65 percent)prepared in a manner similar to Example 1 by reacting commercial Eponresin with diethanol amine were uniformly dissolved in 200 parts of 10percent aqueous solution of malic acid. To this solution were added 400parts of 5 percent aqueous solution of oxalic acid and parts of 6percent aqueous solution of sulfosal'icylic acid. The resulting solutionwas diluted with 146 parts of ion-exchanged water to prepare aresincontaining electrolytic solution.

The apparatus same as in Example 2 was used and the electrolysis wasconducted at 15 V for 10 minutes during which the current density variedfrom 0.6 A/dm to 0.15 A/dm. The bath temperature was maintained at 10 C.After the electrolysis the aluminum alloy was taken out of the cell andbaked at C. for 20 minutes to obtain a continuous and firmly adherentresin coating of a thickness of 20 p.. The anodized surface layerbeneath the resin layer was 0.5 l p. in thickness.

EXAMPLE 6 167 parts of commercial acrylic cathionic resin (made byMitsubishi-Gasukagaku Co.) of a solid content of 60 percent were mixedwith 600 parts of 10 percent aqueous solution of malonic acid. To thissolution were further added 10 parts of 10 percent aqueous solution ofsulfuric acid and 223 parts of ion-exchanged water to prepare aresin-containing electrolytic solution.

An electrolytic cell was divided into two in the same manner as inExample 1 and the larger compartment was filled with the above preparedresin containing solution while the smaller compartment was filled withan aqueous solution containing 6 percent malonic acid and 0.1 percentsulfuric acid. The constant current electrolysis was conducted at acurrent density of 0.5 A/dm for 10 minutes. The bath temperature wasmaimtained at 20 C. The anode was the aluminum alloy panel of Example 1 inthe larger compartment and the cathode was a stainless steel panel inthe smaller compartment. After the electrolysis the aluminum alloy panelwas taken out of the bath, washed with water and baked at C. for 30minutes to obtain a continuous resin coating layer (thickness 5 p.) onthe surface of the alloy panel. The deep yellow anodized layer beneaththe resin layer was about 1' p. in thickness.

EXAMPLE 7 Example 3 was repeated except that the aluminum panel ofExample 1 was used for both cathode and anode and that the electrolysiswas conducted with an alternating electric current at 30 V for 10minutes. There was obtained anodized and resin-coated panel similar tothe product of Example 3;

What we claim is:

l. A method for treating the surface of an aluminous article in a singleelectrolytic bath which comprises conducting the electrolysis with thealuminous article as the anode in said single electrolytic bathcontaining 1. at least one strong acid known per se for the anodizationof an aluminous material;

2. at least one polybasic organic acid; and

3. at least one cationic type resin prepolymer or precondensate in anaqueous system wherein said cationic type resin prepolyme orprecondensate is anionized at least partly, whereby the anodization andelectrodeposition of the resin prepolymer or precondensate are effectedon the surface of the aluminous article in said single electrolyticbath.

2. A method of claim 1 wherein said strong acid (1) is sulfuric acid,sulfaminic acid, sulfophthalic acid or sulfosalycylic acid.

3. A method of claim 2 wherein the concentration of said strong acid (1)in the electrolytic bath is at least 0.05 percent.

4. A method of claim 2 wherein the resin coating after theelectrodeposition is heat-treated to cure the resin.

5. A method of claim 1 wherein said polybasic organic acid (2) is oxalicacid, tartaric acid, malonic acid, citric acid, maleic acid, phthalicacid or malic acid.

' 6. A method of claim 5 wherein the concentration of said polybasicorganic acid (2) in the electrolytic bath is at least 0.3 percent.

7. A method of claim 1 wherein the aluminous article with the resincoating thereon is washed before the heat-treatment with an aqueoussolution containing organic solvents.

8. A method of claim 7 wherein the aqueous solution further contains asmall amount of at least one surfactant.

9. A method of claim 7 wherein the aqueous solution further contains ananti-oxidant.

10. A method of claim 1 wherein the concentratio of said cationic typeresin prepolymer or precondensate in the electrolytic bath is 1' 40percent by weight.

11. A method of claim 1 wherein said cationic type resin prepolymer orprecondensate is thermo-setting.

- a a: a:

1. AT LEAST ONE STRONG ACID KNOWN PER SE FOR THE ANODIZATION OF AN ALUMINOUS MATERIAL;
 2. AT LEAST ONE POLYBASIC ORGANIC ACID; AND
 2. at least one polybasic organic acid; and
 2. A method of claim 1 wherein said strong acid (1) is sulfuric acid, sulfaminic acid, sulfophthalic acid or sulfosalycylic acid.
 3. A method of claim 2 wherein the concentration of said strong acid (1) in the electrolytic bath is at least 0.05 percent.
 3. at least one cationic type resin prepolymer or precondensate in an aqueous system wherein said cationic type resin prepolyme or precondensate is anionized at least partly, whereby the anodization and electrodeposition of the resin prepolymer or precondensate are effected on the surface of the aluminous article in said single electrolytic bath.
 3. AT LEAST ONE CATIONIC TYPE RESIN PREPOLYMER OR PRECONDENSTATE IN AN AQUEOUS SYSTEM WHEREIN SAID CATIONIC TYPE RESIN PREPOLYMER OF PRECONDENSATE IS ANIONIZED AT LEAST PARTLY, SO THAT THE ANODIZATION AND ELECTRODEPOSITION OF THE RESIN PREPOLYMER OR PRECONDENSATE ARE EFFECTED ON THE SURFACE OF THE ALUMINOUS ARTICLE IN SAID SINGLE BATH.
 4. A method of claim 2 wherein the resin coating after the electrodeposition is heat-treated to cure the resin.
 5. A method of claim 1 wherein said polybasic organic acid (2) is oxalic acid, tartaric acid, malonic acid, citric acid, maleic acid, phthalic acid or malic acid.
 6. A method of claim 5 wherein the concentration of said polybasic organic acid (2) in the electrolytic bath is at least 0.3 percent.
 7. A method of claim 1 wherein the aluminous article with the resin coating thereon is washed before the heat-treatment with an aqueous solution containing organic solvents.
 8. A method of claim 7 wherein the aqueous solution further contains a small amount of at least one surfactant.
 9. A method of claim 7 wherein the aqueous solution further contains an anti-oxidant.
 10. A method of claim 1 wherein the concentration of said cationic type resin prepolymer or precondensate in the electrolytic bath is 1 - 40 percent by weight.
 11. A method of claim 1 wherein said cationic type resin prepolymer or precondensate is thermo-setting. 